This invention relates to a method and apparatus for measuring the path length from a reference pint to a target pint using electromagnetic waves.
Various methods and apparatuses have conventionally been used to determine the length of an optical path by measuring the propagation time of light waves. Most of these conventional methods and apparatuses for optical distance measurement use light at a single wavelength. The principle is to measure the propagation time of light wave t, and meteorological conditions along the optical path, such as the temperature T, atmospheric pressure P and relative humidity Rh, then estimate the refractivity N(T, P, RH, . . .) of the optical path, and determined the optical path length D by the equation D={t/(N+1)}.multidot.C, where C is the speed of light in vacuum. Therefore, in order to achieve the correct distance measurement, it is essential to get correct information on meteorological conditions such as the temperature, pressure and humidity of the atmosphere along the path. Correct meteorological measurements can be made under strictly controlled environmental conditions such as in a laboratory or an underground tunnel or if the distance to be measured is short, so in such cases, even conventional methods will suffice since they insure optical distance measurements with a reasonable accuracy.
However, if one wants to measure a distance that extends through the atmosphere for several to several tens of kilometers in a horizontal direction as is often encountered in surveys, a serious problem will arise since it is not easy to estimate the average refractivity of the optical path. This causes an unavoidable difficulty in achieving distance measurement with high accuracy. Attempts have been made to perform precise distance measurements under such conditions by estimating the average refractivity along the optical path in a more accurate way base on the data acquired with many instruments for meteorological observations that are installed along the optical path. However, this method not only requires a large-scale system, but it also give s rise to additional problems such as one involved in calibrating various instruments for meteorological observations. The latter problems would prevent satisfactorily high precision of measurements.
On the other hand, it is known that the refractivity N of an optical path filled with n nonpolar materials can be expressed by the following equation: ##EQU1## where .rho..sub.i and R.sub.i are a density and a wavelength-dependent coefficient of the ith constituent material, respectively. On the basis of this relationship, it has been attempted to totally eliminate the need for meteorological observations by measuring propagation times of a plurality of light waves having different wavelengths through a given optical path.
In a dual-wavelength method, times t.sub.1 and t.sub.2 required for light waves having wavelengths .lambda..sub.1 and .lambda..sub.2 to propagate through an optical path of interest are measured, and determining the relationships: EQU N.sub.1 =.alpha..sub.1 .multidot..rho..sub.s
delete when finished EQU N.sub.1 =.alpha..sub.1 .multidot..rho..sub.s EQU N.sub.2 =.alpha..sub.2 .multidot..rho..sub.s
(where .rho..sub.s is the density of dry air in the optical path, and N.sub.1 and N.sub.2 are refractivities at the wavelengths .lambda..sub.1 and .lambda..sub.2), the optical path length D is calculated as: EQU D=[t.sub.1 +.alpha..sub.1 .multidot.(t.sub.2 -t.sub.1)/(.alpha..sub.1 -.alpha..sub.2)].multidot.C.
Since the density of water vapor int he optical path, namely the humidity, is not considered at all in this method, distance measurements with high precision can hardly be accomplished.
In a three-wavelength method, times t.sub.1, t.sub.2 and t.sub.3 required for light waves having wavelengths .lambda..sub.1, .lambda..sub.2 and .lambda..sub.3 to propagate through an optical path of interest are measured, and determining the relationships: EQU N.sub.1 =.alpha..sub.1 .multidot..rho..sub.s +.beta..sub.1 .multidot..rho..sub.w EQU N.sub.2 =.alpha..sub.2 .multidot..rho..sub.s +.beta..sub.2 .multidot..rho..sub.w EQU N.sub.3 =.alpha..sub.3 .multidot..rho..sub.s +.beta..sub.3 .multidot..rho..sub.w
(where .rho..sub.s and .rho..sub.w are the density of dry air and the density of water vapor in the optical path, respectively, and N.sub.1, N.sub.2 and N.sub.3 are refractivities at the wavelengths .lambda..sub.1, .lambda..sub.2 and .lambda..sub.3), the optical path length D is calculated as: ##EQU2## This is theoretically a very reliable method, but for its implementation, the development of new techniques concerned with a light source, a light transmission control apparatus, a light receiving and detecting apparatus, etc. is indispensable. It is difficult to realize an effective system by the state of the art.